<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        html,body{
            width: 100%;
            height: 100%;
        }
        #app{
            position: absolute;
            left: 0;
            right: 0;
            top: 0;
            bottom: 0;
            margin: auto;

            width: 200px;
            height: 200px;
            background: pink;

            transition: 5s width;
        }

        #app:hover{
            width: 400px;
            height: 400px;
            transition: 5s height;
        }
    </style>
</head>
<body>
    <!--
        浏览器执行代码的速度是极快的
        浏览器渲染页面的速度是相对较慢的
    -->
    <div id="app">

    </div>
</body>
</html>